1. FIELD OF THE INVENTION
The present invention relates to a control apparatus for a brushless motor whose exciting phases are switched over in accordance with a signal of a counter electromotive force (counter e.m.f.) generated in the stator winding, and, in particular, relates to a control apparatus which is improved effectively control the starting operation of the brushless motor.
2. DESCRIPTION OF THE PRIOR ART
A brushless motor has no brush, commutator or the like, and hence its structure is simple, no spark is generated during its operation, and the safety thereof is high. For these reasons, wide application of the brushless motor is expected as compared with a D.C. motor.
In the brushless motor, the switching over of exciting phases is carried out in accordance with a counter e.m.f. in the stator winding. Accordingly, at the time of starting, since the counter e.m.f. is not generated, a rotating magnetic field is supplied externally and forcibly, and after the rotation of the rotor has reached a predetermined speed, the switching over of the exciting phases is carried out by detecting the counter e.m.f. in the stator winding. However, in such a starting method, it takes time until the speed increases, and thus the starting response deteriorates.
FIG. 21 schematically shows the structure of a prior art brushless motor to illustrate a rotational position detecting device. Motor 11 includes a rotor 13 provided with a permanent magnet secured to a rotor shaft 12, and an armature winding 14 is disposed facing the rotor 13. A rotary plate 15 for detecting a rotational position of the rotor 13 is fixed to the rotary shaft 12 for coaxial rotation with the rotor 13, and a detecting element 16 is disposed at a position opposite to the rotary plate 15. In this case, as the detecting element 16, a Hall element, magnetoresistor element, etc. is used, and the position detecting rotary plate 15 which is opposite to the detection element 16 is provided with a permanent magnet having a magnetic pole positioned at a specific rotational angle so that the specific rotational angle position of the rotor 13 with respect to the armature winding 14 can be detected. Alternatively, the detecting element 16 may be composed of a light source and a light receiving element, and then the rotary plate 15 may be formed by a slit plate for transmitting the light at a specific rotational angle position, a reflector for reflecting the light, or the like.
In other words, a position of the rotor 13 relative to a stator is detected by a position detecting device 17 composed of the rotary plate 15 and the detecting element 16, and in accordance with the detected positional relationship, a required exciting current is supplied to the armature winding to control the driving of the motor 11.
FIG. 22 shows a drive control system of the brushless motor 11 having the position detecting device 17 arranged as described above, in which, in accordance with a detection signal indicative of a position of the rotor 13 detected by the position detecting device 17, the ON/OFF switching operation of switching devices of a three-phase inverter 18 is controlled to distribute and supply an exciting current to the three-phase armature winding of the motor 11.
Accordingly, in the control means for such a prior art brushless motor, the position detecting device 17 using the Hall element or the like for detecting the rotational angular position becomes necessary, and thus the arrangement of the driving apparatus becomes complicated. Furthermore, in order to form the position detecting device 17, the Hall element, magnetoresistor element or the like is used, and because of the environment-proof property of such a detecting element, a range of the application thereof is inevitably limited.
A control apparatus which does not require the use of the position detecting element such as the Hall element, magnetoresistor element or the like is disclosed by a paper entitled "P.M. BRUSHLESS MOTOR DRIVES" from the Ninth Annual Symposium of the "Incremental Motion Control Systems and Devices" (held in Champaign, Ill., U.S.A., on June 2-5, 1980), published by Incremental Motion Control Systems Society in 1980. Page 305 of the paper discloses a technique which uses a resistance circuit having resistors respectively connected in parallel with three-phase armature coils which are connected in star-connection, detects variations in the potential difference between a neutral point of the resistance circuit and a neutral point of the armature coils, and controls the switching of an excitation current supplied to the armature coils by means of an inverter circuit in accordance with the detected potential difference variations.
In this case, when a rotating magnetic field is used as the means for effecting the starting operation of a brushless motor, there appears a phenomenon in which, at the beginning of the start of the brushless motor, a reverse rotating torque is developed depending on the relative positional relationship between the rotor and the stator, and, after the rotor has once started to rotate in the reverse direction, the rotor changes its rotating direction to a normal forward rotational direction. Thus, at this time of reversing the rotational direction, rotational oscillation occurs, and this makes it impossible to control the starting operation in a normal manner.
In order to prevent such a reverse rotation of the rotor at the start, an exciting current is supplied to armature coils for particular phases for a predetermined time to fix the rotor at a fixed position at the starting time. Then, the exciting current supplied to the armature coils is switched over by an inverter circuit to cause the rotor to continue rotating. In this manner, the reverse rotating operation of the rotor is prevented, and stable starting can be effected.
However, in the prior art brushless motor as described above, there is a problem in that, at the time of fixing the rotor at a fixed rotational position by supplying an exciting current to the armature coils for particular phases, the rotor once effects rotational oscillation and thereafter it converges to its stable point, and thus it requires time before the rotor becomes stabilized, and also it requires time before the brushless motor completes its starting.